Monday, May 22, 2006

Lecture 016: Chirality Intro

Lecture 016: Chirality Intro

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Jean-Claude Bradley: Chapter five is about chirality, where we're going to be looking at mirror images of molecules.

So, if we start with methane. There are actually a couple of different ways that I can represent a tetrahedron on a two dimensional surface. We saw one of them when we did the dipole moment, and I used a particular way to represent it because it was easy to draw vectors, and to see how they added up or canceled out. But in this chapter I'm going to use a different representation that will serve me better, and that's something I'll try to use consistently, whenever I want to talk about chirality, and that's if you have two bonds, the vertical bonds are in the back of the board, and the horizontal bonds are coming towards you. It's a really convenient way to draw an sp3 hybridized carbon. OK, so there are no bonds in the plane of the board, they are either all coming towards you or away. If you have a model kit that you want to prove to yourself that is the case, you'll see very quickly that a tetrahedron really works out like that.

The reason that it's more useful to use that in this chapter is that we are going to be talking about mirror images, so if I want to take the mirror image of this molecule, we can draw a mirror plane and reflect the atoms across it, and I can see that it's really easy to do that. The hydrogen that is closest to the mirror on the left side will be closest to the mirror on the right side, the one that's farthest away will be farthest away, and the top stays on top and the bottom stays on the bottom.

So by taking the mirror image of CH4 I can see that it's identical. I can take that mirror image, and I can see that obviously it's the same on the left as the right. So CH4 does not have a difference in it's mirror plane, there;s no difference at all.

Lets see, as we start to add complexity to this, how that's going to change.

So if I add a single chlorine, I have chloromethane, and then I reflect everything around it, I can see that again it's exactly the same thing, the left and the right are absolutely identical. I can say that I can superimpose the mirror image on the original molecule, so there is no difference in the mirror images of these two molecules.

So if I add a fluorine to this, I can use the same exercise and see that now the mirror images are also the same.

Now when I add a bromine to this, now something interesting happens. Because when I take a mirror image of that compound on the left I can't superimpose it with it's mirror image. In other words I can't rotate it in any way to make all of the atoms line up. If I try to, for example, rotate it, turn it upside down, to make the Br and the H match up, I'm going to have a mismatch between the Cl and the F. So any two atoms that I line up, are mismatched with the other two. So these are clearly different, they are different molecules, and these are what we call chiral, they are chiral molecules.

We have a new kind of isomer here, they are absolutely identical except in a mirror plane, so that's going to have special consequences that we are going to investigate.

There are tricks to figuring out if a molecule is chiral or not without always having to draw a mirror plane. One of the things you'll notice is that in all the four examples I did, this is the first example that I had four different groups on an sp3 hybridized carbon. So it turns out whenever you have four different groups on an sp3 hybridized carbon, you're going to have a chiral center, and the molecule will be chiral. So that's four different groups on sp3 carbon.

[inaudible question]

Jean-Claude Bradley: OK, let's take a look at that..So what do you want to do, the top and the bottom are that same?

So, are those two molecules superimposable? They are superimposable because I'm allowed to rotate any way I want. And another really convenient way of drawing molecules this way, is that it enables me to rotate 180 degrees without moving groups in and out of the plane. So if I take the second molecule that I drew, and then if I rotated it 180 degrees, what will happen? The top H will go to the bottom, the bottom H will go to the top, and the F that's on the right would now go to the left, and the bromine would go to the right. So in this case, you could have predicted that easily by seeing I've got two hydrogens, I've got two groups that are the same, therefore this is not chiral, end of story.

[inaudible question]

Jean-Claude Bradley: Well the way that you drew this projection, you can't flip the groups in and out of the board, because then you're never going to be able to superimpose them. So really the only thing that you can do is to start to rotate, and keep everything the same, then your bonds are going to line up every time.

Anything else?

So that's basically a little introduction to chirality. So let's also give a definition here. When I do have a chiral molecule- remember the definition, that a chiral molecule is one where you cannot superimpose it's mirror image. Now when I'm talking about determining a chiral center, a chiral center is one in which I have four different groups on an sp3 hybridized carbon. So those are the little subtleties, and we'll see actually how those subtleties will play out when we have multiple chiral centers. But the only definition that counts for a chiral molecule is taking the mirror image, that's it. If you can't superimpose it it's definitely chiral.

Ok, so now we have a set of isomers that are new. They are different from the structural isomers, they are different from the geometric isomers that we saw with the cis and trans. They have a special relationship, these two isomers, they are called enantiomers. So to have an enantiomer all I have to do is have a mirror image of a molecule that is not itself. That's the biggest challenge when you're doing these problems is, if you have two molecules drawn and they are mirror images they can either be enantiomers or they could be the same molecule and you haven't rotated it carefully to make sure that it's not the same molecule.

Enantiomers have very similar properties, because if you think about it, if the only difference is in a mirror plane, then all the bond lengths are going to be the same, all the bond angles will be the same, and therefore all of the energies are going to be the same. So if I have one enantiomer and it's pure, and I'm determining a melting point or a boiling point, it's going to have absolutely the same properties as it's enantiomer. Because that's it, if they are just living in mirror worlds they are going to have the same melting point, the same boiling point, and a lot of the same physical properties.

So they are going to have the same properties unless they are interacting with something else that's chiral, and that happens actually quite a bit. It's not going to happen if I take a Bunsen burner, heat is not chiral. But there are some things that are chiral. For example, a lot of drugs have chiral centers, and if you inject one enantiomer, it's going to have a completely different effect than another enantiomer. The reason for that is that we are chiral. All our proteins are chiral, everything, basically, that's biological is based on some chiral building blocks. So when you have the molecule that's coming in and it's got to fit into a receptor, that receptor is chiral. So that's why it;s very rare that you are going to have enantiomers with drugs that are going to have the same effect, because they have to fit into a receptor. So there's one area that they are definitely going to be different.

Another area that they are going to be different is when they interact with plane polarized light. So let me draw a little picture here of how we set up this experiment.

So if we take light, and we pass it through a polarizing filter. If you have sunglasses it's probably a polarizing filter. It's basically a filter that has tiny grooves that are all oriented in one direction. And what happens is that, just random white light is composed of an electric field vector and a magnetic field vector, it's going up and down, and it's going up and down in all directions. When you have a polarizing filter, it filters out only those waves that are going in one direction, that are going up and down. So if I have a filter like this, all the lines are vertical, if the light is undulating in other directions it's not going to make it through, so I end up with light that is called plane polarized light after it passes through a polarizing filter.

Now if I have a solution that contains a chiral compound that just contains a single enantiomer, that light is going to interact with that compound by turning either to the right or to the left. So this is a solution... I have a chiral molecule, I have a solution, it goes through and now it either goes to the right or to the left. And then, we're going to hit another filter. So in this example I rotated the filter to the right by a certain angle. So if the light let's say got rotated by ten degrees. As I rotate this filter, when I hit ten degrees I'm going to get maximum output of the light. If I'm perpendicular to the direction of the plane polarized light, it's going to turn black. As I turn it closer and closer to the maximum angle it's going to become lighter and lighter and I can tell when actually it matches up. SO if it rotates it by ten degrees I can tell that by having a second filter and seeing how much I have to rotate the filter to let through most of the light.

All right, and then I have here a detector. So if you want a prettier picture than this take a look at you book, there's a nice drawing of all this, but this is just the concept, Ok? You've got plain polarized light, you have your solution, it will rotate it by a certain amount, and I know that by the detector that I put here.

So what we can say about chiral molecules and enantiomers, if we go back here, we can say that enantiomers will rotate plane polarized light in opposite directions. And by the same amount, and there will be the same concentration on the same equipment.

So if we have one enantiomer that rotates negative ten degrees, we know that it's mirror image will rotate plus ten degrees. That's a property that all enantiomers are going to have. Now you can't tell by looking at a molecule whether it's going to rotate it to the left or to the right, or by how much. We can't tell that, but we do know that they will definitely rotate it by some amount.

So how do we actually standardize this so that we can compare... If you make a molecule and you want to see if it's the same thing that someone else has made you can have your polarimeter, you can put it in your polarimeter and it will rotate it by a certain amount, but if you don't do the experiment the same way how can you compare your results? So in order to standardize all this so that any chemist will be able to get the same results we need to quantify certain aspects of this experimental design.

So the first thing we have to worry about is C which is the concentration, and then we have L, which is the length of the tube in which you have your solution, and then basically you have the rotation that you measure, so you have the angle here. What I can do is write a formula here. So alpha OBS is your observed rotation on the machine, and the alpha that's inside square brackets, that's called specific rotation. The specific rotation is the parameter that is going to be listed in books. So it doesn't matter what your equipment was like, if it had a different path length, all that is accounted for in our formula and everyone will get the same number.

Basically what it tells you is that if you double the concentration, you're going to double the angle of the rotation, which would make sense, and if you make the path twice as long you're going to double the rotation, so it's all linear and it all adds up.

The 25 refers to room temperature, it's 25 Celsius and the D is the D line of sodium because we need to specify a certain wavelength that we are measuring. The specific rotation will vary depending on the temperature a little bit and it's going to depend on the wavelength you use to do the measurement. SO to make sure everything is taken into account we usually write down the D line of sodium.

So obviously if the specific rotation of one enantiomer is plus ten degrees its enantiomer will be minus ten degrees. That's all that it means.

So when we say that a compound can rotate plane polarized light to the left or to the right we say that it has optical activity, and a compound that will rotate the light to the right will be called dextrorotatory, and the opposite would be laevorotatory. So in any pair of enantiomers you will have one that will be dextrorotatory and one that will be laevorotatory. Sometimes when you see the naming of the compounds dextrorotatory will be denoted as a small d, laevorotatory will be a small l. You can also see a plus in parenthesis to show that it rotates plane polarized light to the right, and a minus. So those are ways of actually specifying the properties.

So I think this is a good place to stop, and I'll hang around if you want to discuss anything.

Transcription by CastingWords


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